In one aspect, the present invention relates generally to a remote tire monitoring system and components for such a system. In other aspects, the present invention relates to methods and apparatus for automatically wireless autolocation for tire monitors in such a system.
Systems have been developed to monitor a characteristic such as tire pressure of a vehicle and to report the characteristic to a receiver at a central monitoring station using radio transmissions. A monitor is located at each tire and periodically takes a measurement of the tire characteristic. The tire monitor senses activity through action of a roll switch. The roll switch is a mechanical switch using a ball and spring to sense when the vehicle is at rest or is in motion, and to measure tire characteristics accordingly. The monitor then transmits the results of the measurement in a radio frequency transmission to the central monitoring station which produces an alarm or a display in response to the measurement.
One problem with such systems has been the need to program the location of the transmitters at the central station. To be fully useful, the tire characteristic data is preferably associated with the tire which originated the measurement when presenting a display or alarm. Each monitor includes identification information which can be transmitted with the measurement. The tire monitor is preferably activated to produce this information and the information is then conveyed to the central station and associated with the position of the tire.
In the technique of U.S. Pat. No. 5,600,301, the tire monitors each include a reed switch or other magnetic device. A magnet is passed near the reed switch, causing the monitor to transmit a radio frequency transmission that includes identification data. A service technician repeats this process at each wheel and then loads the identification and position information into the central monitoring station. Another method provides a printed bar code on each tire monitor which contains the identification information and which may be read with a suitable bar code reader.
In U.S. Pat. No. 5,880,363, an activation signal is provided from the central controller to a low frequency transmitter at each wheel well. The transmitter generates a low frequency signal to activate the tire monitor. The tire pressure monitor responds by generating a long wave identification signal and transmitting that signal with tire pressure and identification data directly to the control unit. The long wave identification signal is used to identify the position of the tire by distinguishing this transmission from other transmissions received by the controller.
U.S. Pat. No. 5,883,305 discloses two-way communication of data by radio signals. A tire pressure monitor is activated by a radio frequency signal transmitted by an antenna in the wheel well adjacent the tire. The tire pressure monitor transmits a second radio frequency signal which is detected by the wheel well antenna. The second signal is demodulated to detect that tire pressure data.
U.S. Pat. No. 6,204,758B1 discloses a tire monitor including a tangential accelerometer for detecting acceleration of the tire monitor. Tangential acceleration along a single axis is detected to determine acceleration. Position information for the tire monitor is determined in response to the acceleration.
These previous techniques have been limited in effectiveness. The magnetic programming technique may be subject to interference and crosstalk, for example in a factory where many such tire monitors are being assembled with tires and vehicles. The bar code label system requires a label at each tire which can be lost or become dirty or illegible. The apparatus for transmitting a long wave activation signal and generating a long wave identification signal therefrom is too expensive for some applications. The two-way data communication technique requires demodulation of the received radio signals at the wheel well and coaxial cabling back to the central controller, both of which add to the cost of the system. The tangential acceleration which must be sensed requires a highly sensitive accelerometer which may be prohibitively expensive in the current application.
A further limitation of some of these prior techniques is the manual operation requiring activation by a service technician. A system is desired which automatically conveys wheel position data to the receiver. Such a system would be particularly useful after any change in tire position, such as tire rotation or replacement of a tire.
U.S. patent application Ser. No. 09/557,682, commonly assigned with the present application, discloses a system and method in which tire monitors are located at each wheel of the vehicle and periodically transmit tire data along with a tire monitor identifier. Four small, inexpensive RF detectors are located near each wheel. Each RF detector is connected to the central control unit by a power line and a ground line. When a tire monitor transmits data by emitting an RF transmission, the RF detector that is closest to the transmitter will detect the burst of RF energy. The RF detector responds to the RF energy by modulating the power line to the control unit with the envelope of the transmitted data. The control unit detects this modulation on one of its power lines. Also, the RF receiver of the control unit receives and demodulates the data transmitted by the tire monitor. The control unit associates the received data with the position indication provided by the modulation on the power line. When the positions of the wheels on the vehicle are changed, the control unit can determine the new position using the modulated power line in association with the tire monitor identifier in the transmitted data.
While this system has been very successful in application, a system and components featuring reduced cost and improved durability is desired. The roll switch included in prior tire monitors is necessarily subject to a lot of forces during use, including high-g forces during vehicle motion and shock and vibration as the tire traverses rough pavement. Also, the roll switch tends to degrade in performance over time and become a non-ideal switch. Still further, presently available roll switches are not true surface-mount devices of the type that can automatically be assembled on a printed circuit board, and the presently available roll switch is relatively expensive. Accordingly, there is a need for an improved tire monitor and remote tire pressure monitoring system using such improved tire monitors.